The present invention relates to the field of heavy lifting and handling, and more particularly to a traction system using a cable including a plurality of substantially parallel tendons movable for pulling a load.
In certain configurations, it may be necessary to arrange for some angular deflection of the traction cable, for example for pulling over an obstacle and/or to provide sufficient leverage to carry out the lifting or tensioning operation. Depending on the configuration, the deflection angle of the cable may be constant, or may vary while the load is moving.
When the traction cable is made of parallel tendons, e.g. strands, their arrangement in the cross-section of the cable must be controlled to avoid undesired transverse contact stresses between the tendons which hinder transfer of the traction forces to the load and may damage the tendons.
It is also desirable to balance the tensile forces between the multiple tendons. Otherwise one or some the tendons take up most of the efforts, which is detrimental to the cable capacity and durability.
A deflection angle of the multi-tendon traction cable is problematic to meet these requirements. Where the cable is deflected, some of the tendons typically have a larger radius of curvature and these tendons tend to undergo larger tensile forces and to be pressed against the other tendons on the inner side of the curvature.
Some deflections systems use pulleys to reduce friction efforts. Such a solution may be difficult to implement where the tendons of the cable are arranged in multiple layers. It is incompatible with certain pulling operations, especially when very high traction forces must be applied, for example where a very heavy load (e.g. a ship or a construction work) must be lifted, lowered or dragged, where a structural prestressing or load-bearing cable must be tensioned, etc. Such very high traction forces would require extremely sturdy pulleys and excessive friction and stress would be generated at their axles and bearings.
An object of the present invention is to provide another solution which is better suited, in particular to pulling operations with very high traction forces applied to multi-tendon cables.